uwb: use for_each_clear_bit()

Use for_each_clear_bit() to iterate over all the cleared bit in a
memory region.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Stefano Panella <stefano.panella@csr.com>
Cc: David Vrabel <david.vrabel@csr.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>